Vehicles such as electric scooters and electric vehicles which do not use an internal combustion engine are under development in these years. The motor used as a drive source for such vehicles is protected by being housed in a metallic casing.
Such a motor casing is proposed to have a structure as shown in FIG. 5.
A motor casing 100 in FIG. 5 mainly consists of a case 101 and a cover 102, and it is mounted on a vehicle body (not shown) via the bottom end of the case 101.
The case 101 and the cover 102 are joined into one body with a sealing member 106 between them by tightening steel bolts 104 in several points. And, a stator 105 of the motor has its outer periphery held to be fixed between the case 101 and the cover 102. In the drawing, reference numeral 5 denotes a rotor, 9 a ring-shaped circuit board, 10 a rotor position sensor, and 11 a screw for fixing the circuit board 9.
Considering that the motor housed in the casing 100 is used outdoors, the casing is needed to have waterproofing and dustproofing functions in order to prevent rain water or dust from entering into the casing.
Therefore, the above-configured casing 100 is needed to be designed to place the sealing member 106 on a joining portion between the case 101 and the cover 102.
The sealing member 106 may be an O-ring formed of an elastic material or a liquid gasket to be applied.
When the O-ring is used, it is placed to be pressed and deformed on the entire circumference of the joining portion between the case 101 and the cover 102, thereby filling a gap between them by the O-ring. Therefore, it is necessary to form a groove, in which the O-ring is inserted, on the entire circumference of the joining portion between the case 101 and the cover 102. But, there are disadvantages that it is not easy to form the groove accurate in size and the cost becomes high. And, sealing by the O-ring might be incomplete due to its dimensional error, and sealing properties of the O-ring is lowered because the O-ring which is pressed and deformed is deteriorated due to its change with time.
And, when the liquid gasket is applied, a predetermined amount of liquid gasket is applied and hardened on the joining portion of the case 101 and the cover 102, so that the gasket fills a gap between them after assembling. Therefore, a step of applying the liquid gasket, a step of hardening the applied liquid gasket and the like are required, making the assembling troublesome. Thus, workability is poor. Besides, an excess portion of the liquid gasket may flow into the casing to adhere to the components of the motor. Therefore, it is necessary to strictly control the viscosity and applied amount of the liquid gasket.
After applying the liquid gasket, the case 101 and the cover 102 are connected by tightening the bolts 104 at several points. If the respective bolts are not tightened by an equal tightening force, it happens often that the cover 102 is slanted against the case 101 causing a gap in the liquid gasket and crippling the complete sealing. Therefore, the bolts 104 are needed to be tightened with extreme care, and the tightening work is troublesome.
In view of above, the conventional casing 100 has the joining portion exposed to the outside surface and needs a member dedicated for securing waterproof and dustproof, being disadvantageous in view of production and assembling. And, it has a disadvantage that it is disassembled easily by the user or the like.
Therefore, a primary object of the present invention is to provide a motor casing which has its waterproofness and dustproofness improved and the housed motor reliability enhanced.
Since the casing 100 described above has the flange 103 which is formed on the cover 102 is close to the top end of the cover 102, the bolts 104 are not longer than they are required. Their length is about half or less of the length of the stator 105 in the direction of a motor rotatable shaft 51.
And, since the component material of the bolts 104 is different from that of the case 101 and the cover 102, they have a different thermal expansion coefficient.
Therefore, if a temperature change is caused owing to heat generation by driving the motor, the bolts 104 may have a change in axial force due to a thermal stress between different types of materials, causing an excessive tensile force onto the bolts 104 or loosening them. In other words, a difference between the thermal expansion of the bolts 104 and that of tightening members (the bolts 104 and the case 101) and the tightened members (the stator 105 and the cover 102) increases, resulting in increasing a change rate between them. Therefore, the bolts 104 cannot keep the initial axial force.
Conventionally, bolts had a high strength (high allowable stress) to deal with the change in bolt axial force, their tightening torque was increased, and a large number of bolts were used to increase the number of bolt tightening points.
Therefore, the conventional case fixing structure had a disadvantage of involving the increase of parts cost. And, the high strength bolts had a disadvantage that their corrosion resistance is low because they could not be plated (corrosionproof plating) because of hydrogen embrittlement, and therefore the bolts had a poor corrosion resistance. Besides, since the bolt tightening torque is high, the tightening work is not easy, it takes time to assemble the casing, and workability is poor.
A second object of the invention is to provide a method for producing a motor casing in that in selecting coupling members which are affected by a temperature change, a strength classification of the coupling members required is determined from a numerical value in a predetermined range which is decided in view of the dimensions of the casing and the coupling members, so that the coupling members can be selected and assembled with ease.